Conventionally, a pin-through-hole technology (PTHT) is employed to mount electronic components onto one side of a printed circuit board through pin-through-hole connections. Since a plurality of through holes corresponding to the pins of the electronic components are drilled in the printed circuit board and the solder joints between the electronic components and the electronic components are very large, the pin-through-hole technology may be applied to the relatively low-density integrated circuits.
With increasing progress of fabricating integrated circuits, the semiconductor packages are developed toward minimization and high integration. As a consequence, the requirement of increasing the pin density of the semiconductor packages becomes more important. Recently, a surface mount technology (SMT) is gradually used to constructing relatively high-density integrated circuits. The printed circuit board usually has flat solder pads without holes. Generally, the process of performing the surface mount technology (SMT) principally comprises the following steps. First of all, solder paste is applied to all the solder pads. Then, the surface-mounted devices (SMDs) are precisely placed on the solder pads. The SMDs and the printed circuit board are then heated in a reflow soldering oven in order to minimize thermal stresses when the assemblies cool down after soldering. The printed circuit board then enters a zone where the temperature is high enough to melt the solder paste, thereby bonding the SMDs' pins on the printed circuit board.
Since the SMD components may be mounted on both sides of the printed circuit board, the space utilization of the printed circuit board is increased. In addition, the SMD components are usually made physically small and lightweight, thereby allowing much higher circuit densities. Surface mounting lends itself well to a high degree of automation, so that the fabricating cost is reduced.
Due to the very small size and spacing of the SMD components, there are also some drawbacks. For example, the alignment of the SMD components with the corresponding solder pads on the printed circuit board may usually be oblique or shifted if the profiles of the solder pads are improper or the solder paste is not evenly applied. In addition, a solder paste, which exhibits excessive out-gassing during the initial stages of the melting of the solder powder, will also result in tombstone defects. In a case that the SMD components fail to be well bonded on the corresponding solder pads, the electrical properties of the integrated circuit are deteriorated.
FIG. 1A is a schematic cross-sectional view illustrating a SMD component bonded on conventional solder pads. FIG. 1B is a top view of the resulting structure of FIG. 1A. As shown in FIGS. 1A and 1B, the solder pad 12 has a substantially rectangular profile and arranged on a surface of a circuit board 11. A surface-mounted device (SMD) component 10, for example a passive component such as a stack capacitor, has a first conductive part 101 and a second conductive part 102. The SMD component 10 is bonded on the solder pads 12 via solder paste 13. Generally, the width of the solder pad 12 is greater than the width of the SMD component 10. Ideally, the length direction of the SMD component 10 is consistent with a straight line passing through the centers of the two solder pads 12. Since no marks or retaining elements are arranged on the solder pads 12, the alignment of the first conductive part 101 and the second conductive part 102 of the SMD component 10 with respect to the solder pads 12 is usually unsatisfied. For example, the length direction of the SMD component 10 is deviated from the ideal line, as is indicated in the dashed region. Under this circumstance, the electrical properties of the integrated circuit will be deteriorated and the circuit board 11 is not aesthetically pleasing.
As known, during the solder paste 13 is liquefied and then cooled, the solder paste 13 exerts small amounts of torque on each side of the SMD component 10 through surface tension and cohesion. If the solder paste 13 is not evenly applied, the differences in cohesion cause more torque on one side that pulls the opposite side up and off the solder pad 12, as can be seen in FIG. 2. Therefore, a tombstone defect is resulted and the SMD component 10 is no longer electrically connected to the circuit board 11.
Since electronic products become more diverse, the electronic components used in these electronic products have a variety of specifications. In other words, the specifications of the solder pads are varied depending on the kinds of the corresponding SMD components. Therefore, the solder pads are not cost-effective.
In views of the above-described disadvantages resulted from the conventional method, the applicant keeps on carving unflaggingly to develop a universal solder pad according to the present invention through wholehearted experience and research